1. Field of the Invention
The invention relates to a highly selective filtering device for use particularly in transmission systems, and also to the filtering method used.
2. Background Information
Transmission systems use very many types of filters. By way of example, FIG. 1 shows an example of an external unit 1 connected between an antenna 2 and an internal unit (not shown) such as a satellite decoder with a return channel or a decoder for a high-speed point-multi point transmission system. The external unit 1 is of a type which requires high precision in its local oscillator 9, as described for example in U.S. Pat. No. 5,844,939. Two bandpass filters 3 and 4 are used to separate the transmission and reception bands. Two other bandpass filters 5 and 6 are used to separate the intermediate transmission and reception bands. Two mixers 7 and 8 and an oscillator 9 carry out frequency transpositions between the transmission and reception bands and the intermediate transmission and reception bands. To provide high precision in the frequency of the oscillator 9, the latter is locked to a reference carrier with a highly stable frequency by means of a phase-locked loop which additionally comprises a filter 10 which selects the intermediate band reference carrier, a phase comparator 11 and a precise reference oscillator 12. A person skilled in the art can find further details of this external unit in U.S. Pat. No. 5,844,939.